Automatic stoking mechanism for electric furnaces and method of operation



e 4, 1957 M. 0. SEM EIAL I 2,794,843

AUTOMATIC STOKING MECHANISM FOR ELECTRIC FURNACES AND METHOD OF OPERATION 2 Sheets-Sheet 1 Filed Dec. 23, 1955 lNVEN l'OR5 w Earl Lara/6 Mai/Has 0. 667m M ufin w By gr, ATTORN Y S June 4, 1957 M. 0. SEM ETAL 2,794,843

AUTOMATIC STOKING MECHANISM FOR ELECTRIC FURNACES AND METHOD OF OPERATION Filed Dec. 23, 1955 2 Sheets-Sheet 2 800 x u Q 700 f u k I E 600 v w: *5 g 500 H A TIME IN HOURS INVENTORS Karl [arc/6 fl/azfiz'ew 0, 'efii/ 5y de/MLL M ATTORNEY United States Patent AUTOMATIC STOKING MECHANISM FOR ELEC- TRIC FURNACES .AND METHOD OF OPERA- TION Mathias Ovrom Sein, S mesta'd, Oslo, and Karl Lorck, Ora, Oslo, Norway, asiguors to Electrokemisk A/ S, Oslo, Norway, a corporation of Norway This application relates to a method of mechanically stoking an electric furnace (particularly an open furnace) of the type in which an electrode is used, with the charge coming up a substantial distance around the electrode. There is also shown a method of indicating when stoking should take'place and automatic means for causing stoking to be carried on.

This invention can best be understood by reference to the accompanying drawings in which:

Fig. l is a vertical section through a furnace equipped with an automatic stoker;

Fig. 2 is a plan view of the structure shown in Fig. 1, and

Fig. 3 is a diagram showing the relationship of furnace temperature to the need of having thefurnace stoked.

Electric furnaces of the reduction type usually operate with the electrode dipping into the solid charge. Smelting takes place mainly around thlower part of the electrode where a. crater is formed into which the charge sinks during smelting. The form and size of the crater WillWM'Yin-the different smelting. processes and will depend on the furnace load, the position of the electrodes, etc. -.As a ruletthe crateris narrow at the top and widens towards the bottom as indicated in the drawings, which are intended for the purpose of illustration to show conditions in a smelting furnace for the production of FeSi and other ferro-alloys.

To obtain a uniform and elfective operation of the furnace it is of great importance that the charge shall sink evenly and smoothly into the smelting zone in the lower part of the crater. In many, processes, as for example in the production of FeSi, crusts and bridges of sintered material tend to form in or above the narrow part of the crater. The charge smelts below these bridges and thereby cavities are formed which will fill up with furnace gas. The pressure of this gas will gradually increase and finally the gas may penetrate the charge suddenly with an eruption of an explosive nature. In this case it is said that the furnace blows. This eruption, which may be rather violent, causes substantial losses of material and energy and creates substantial strains on the furnace equipment. To prevent this occurring, it is necessary to stoke or work the charge so as to make it sink evenly into the smelting crater and prevent the formation of bridges.

Stoking is especially necessary immediately around the electrodes, and in accordance with this invention stokers are supplied surrounding each electrode and so suspended that they may be moved up and down vertically to agitate the charge.

In the drawings, designates the furnace, 12 is the electrode, 14 is the usual electrode holder with the electrical connections and suspension mechanism not shown, as they are well understood in the art, and 16 indicates the actual stoking member which in this case is shown as a cylinder surrounding the electrode. The cylinder is somewhat larger than the electrode and preferably is 2,794,843 Patented June 4, 1957 sealed against the electrode or electrode holder as indicated at 18 and thus serves to catch any gas that may go up around the electrode. At the same time due to its being substantially larger than the electrode, it will contact the charge in the furnace at a distance somewhat removed from the electrode. The charge here is indicated by the numeral 18. It is intended that the cylinder 16 shall be movable up and down to agitate the charge around the electrode periodically as needed.

As shown in the drawing, the bottom of the cylinder 16 is preferably beveled, with its outer circumference coming down lower than its inner diameter. This beveled face will act as a plow when the cylinder moves up and down thereby forcing the charge inwardly toward the electrode.

Framework 20 runs across the furnace on a level with the top of the cylinder 16 and supports this cylinder. Normally the framework 20 rests on the jacks 22 which are adjustable to raise and lower the casing as needed. Beneath the ends of the framework 20 are shafts 24 carrying eccentrically mounted cam wheels 26. A motor 30 drives the shafts 24 when it is desired to stoke the furnace. As the shafts 24 rotate, the cam wheels 26 strike the lower faces of the framework 20 and lift and agitate this framework and thereby raise and lower and agitate the cylinder 16 so that the crusts around the electrode and near the cylinder are broken.

Since gas will collect within the cylinder 16, a gas exhaust pipe 32 is supplied for withdrawing gas from within the cylinder. It is understood that the cylinder 16 should be water-cooled and the water connections are indicated at 34.

By thus providing for vertical movement and agitation of the cylinder 16 which surrounds the electrode but is spaced from it, particularly efiicient stoking can be had;

at the point where it is most needed.

With a good steady furnace operation, the evolution of gas in the furnace will be quite even and the gas will give off a greater part of its heat to the cooler, upper portion of the charge and will thereby be substantially cooled. As already brought out, if the charge does not sink down continuously, bridges will tend to be formed under which the gas will collect and eventually the gas will break through the sintered material with great force.

When the gas thus rushes through rapidly, less heat will be given off to the charge and if means is provided for measuring the temperature of the gas, it will be found that a marked rise of gas temperature occurs. This serves as an important indicator of the furnace conditions and the operation of the stoker may be controlled by such temperature. For example, a thermostatic switch may be provided in the pipe 32 as indicated at 36 and this may be connected to motor 30 to start up the motor when the gas temperature reaches a predetermined level. It is to be understood that after the motor is started up it will automatically shut off after a predetermined time interval. The stoking should not be carried on too long, as the charge may then be packed too tightly, which will result in a reduction of the porosity of the charge, thereby increasing the electric conductivity and causing the electrode to assume a high position. This in and of itself may cause the gas temperature to rise.

The relationship of the gas temperature to the operation of the furnace is an exceedingly important function and using a variation in temperature as an indicator or control for the time of stoking the furnace makes for particularly etficient operation of the furnace. For example, in most smelting operations temperature will usually range between about 300 and 800 C. The normal temperature usually within this range can readily be determined. When there is a substantial rise above the 3, normal temperature, it indicates that conditions in the smelting crater are abnormal, and therefore when such rise occurs, stoking should take place.

In Fig. 3 we show a temperature curve taken in connection with a smelting furnace for the production of 45% FeSi. In this diagram the temperature of the gas starts at 500 C. and rises to about 850 C. It is understood that no stoking has been done during this period. At about 850 C. stoking was started and the charge was pushed into the smelting craters, readjusting the gas flow to normal. The gas temperature then immediately sank to about 600 C. but rose again during the continued operation. In the diagram, minor peaks .are shown between the main peaks. These indicate times when bridges started to form but fell of their own weight before stoking started. By having stoking take place as soon an abnormal temperatureis reached, the operation of the furnace can be maintained at maximum efficiency.

Of course, this control is more important in a closed furnace than in an open furnace but even in an open furnace a continuous temperature record of the furnace gases will give a good picture of how the furnace has been looked after.

While we prefer to measure the temperature of the exhaust gases directly, it is to be borne in mind that the temperature of the exhaust gases will also alfectthe temperature of the cooling water that is withdrawn from within the cylinder 16 and therefore measuring the temperature of this water will also serve as a measure of the temperature of the gases. Further, by measuring the temperature of water withdrawn from ditferent parts of the cylinder, the operations of the furnace in different areas can be determined.

What we claim is:

1. In an electric furnace of the type in which the charge surrounds the electrode for a substantial depth, a stoking apparatus comprising a cylinder surrounding the electrode with a clearance between the two to form a gas chamber, means for forming a seal between the upper part of the cylinder and the electrode which permits relative vertical movement to take place between the two, means for withdrawing gas from within said cylinder and means for agitating such cylinder so that crusts and bridges in the, charge in the area of the lower part of the cylinder maybe broken by the cylinder.

2. A structure as specified in claim l which further includes means for measuring the temperature of the gases collected in the cylinder.

3. A structure as specified ,inclaim 1 in which the means for agitating the cylinder' is motor-driven.

4. A structure as specified in claim 2 in which the means for agitating the cylinder is motor-driven and which includes temperature-sensitive means positioned to be acted on by gas withdrawn from within the cylinder to start the motor for agitating the cylinder when the temperature of said gases rises-above a predetermined level.

5. The method ofoperating an electric furnace of the type in which the charge surrounds the electrode for a substantial depth, which comprises the steps of collecting from around the electrode gases generated within the furnace, continuously measuring the temperature of such gases and causing the furnace to be stoked when the temperature of such gases rises above a predetermined level.

6. A method as specified in claim 5 in which the I stoking of the furnace takes place automatically when the heat of the gases rises above a predetermined level.

7. The method as specified inclaim 5 in which the temperature of the gases is measured indirectly by maintaining such gasesin contact with a water jacket and measuring the chang'es inl itemperature of water withaw f ai"s st-;

References in the file of patent UNITED STATES PATENTS Mi de utne Oct. 21, 1930 1,830,992 Frenzel* -Nov.'1o, 1931 2,473,681 Hansen June 21, 1949 

1. IN AN ELECTRIC FURNACE OF THE TYPE IN WHICH THE CHARGE SURROUNDS THE ELECTRODE FOR A SUBSTANTIAL DEPTH, A STOKING APPARATUS COMPRISING A CYLINDER SURROUNDING THE ELECTRODE WITH A CLEARANCE BETWEEN THE TWO FORM A GAS CHAMBER, MEANS FOR FORMING A SEAL BETWEEN THE UPPER PART OF THE CYLINDER AND THE ELECTRODE WHICH PERMITS RELATIVE VERTICAL MOVEMENT TO TAKE PLACE BETWEEN THE TWO, MEANS FOR WITHDRAWING GAS FROM WITHIN SAID CYLINDER AND MEANS FOR AGITATING SUCH CYLINDER SO THAT CRUSTS AND BRIDGES IN THE CHARGE IN THE AREA OF THE LOWER PART OF THE CYLINDER MAY BE BROKEN BY THE CYLINDER. 